Method for constructing arc discharge devices



July 17, 1962 J. M. MORGAN ETAL 3,

METHOD FOR CONSTRUCTING ARC DISCHARGE DEVICES Filed Jan. 24, 1961 2Sheets-Sheet 1 J N M.MORGAN J EPH'B.KUBECK INVENTORS 3,045,093 METHODFOR CONSTRUCTING ARC DISCHARGE DEVICES Filed Jan. 24, 1961 July 17, 1962J. M. MORGAN ETAL 2 Sheets-Sheet 2 JOHN M. MORGAN JOSEPH B.KUBECKINVENTORS BY @ZI ATTORN EY 3,045,093 METHOD FOR CONSTRUCTIN G ARCDESCHARGE DEVICES John M. Morgan, Scranton, and Joseph B. Knbeck,Taylor, Pa, assignors to General Electric Company, a corporation of NewYork Filed Jan. 24, 1961, Set. No.184,660 7 Claims. (Cl. 219-95) Thisinvention relates to are discharge devices and more particularly, to amethod for constructing triggered spark gap tubes.

' The triggered spark gap is an arc discharge device that functions as ahigh voltage switch; it employes the electrical breakdown of a gaswithin the tube in order to provide conduction. Triggered spark gapshave been used for switching up to at least 30,000 volts. They areespecimly useful in electrical circuits that require extremely fast andvery reliable switching functions. Typical applications are thosewherein switching times must be less, and often considerably less, thanone microsecond. One such application of the triggered spark gap isdisclosed in the United States Patent No. 2,867,728 to H. C. Pollock forLogging Apparatus, which issued on January 6, 1959. Shown therein is acircuit wherein the spark .gap is utilized for pulsing a source ofneutrons used for litho-logical logging analyses. The specificrequirement of the logging apparatus of that patent is for a highvoltage pulse which is a small fraction of a microsecond.

'In essence, the triggered spark gap is a tube, the interior of which isfilled with a gas which is capable of ionization when an appropriatevoltage is applied between certain of the electrodes. The triggeredspark gap tube comprises three electrodes. Two of them act as thecontacts of a switch, and the third acts to close this elemental switchby virtue of breaking down or ionizing, with a preparatory or initiatingpulse, the gas disposed between the third electrode and one of the othertwo electrodes. The two electrodes which act as the contacts of switchare called dome electrodes because they are hollow hemispherical shapes.The dome electrodes are disposed opposite one another, and are spacedapart (and insulated from each other) by a distance which is appropriatefor the voltage of interest. The third electrode (which is shaped like atube or rod and is called the trigger electrode) projects just through,and is insulated from, the surface of one of the two dome electrodes.When the spark gap is to be switched, a short pulse is applied betweenthe trigger and one of the two dome electrodes.

This difference of potential is sufiicient to break down the gas betweenthese two closely spaced electrodes, where by ionization takes placetherebetween. One of the theories advanced to explain the triggering ofthe triggered spark gap is that the ultraviolet radiation which isgenerated by this localized breakdown serves to trigger the ionizationof the rest of the gas between the two dome electrodes. This results inan avalanche breakdown or ionization of the gas between the domeelectrodes. The ionized gas serves as a conductor thereby to efiectivelyclosethe switch. The term breakdown as applied to spark gaps meanselectrical conduction of the gas between the electrodes. In this way,the triggered spark gap actsas a high voltage switch.

Three of the most important conditions for successful operation of aspark gap should be understood for this discussion. The first conditionis that breakdown (i.e., conduction) must not occur as long as thepotential ap plied between the dome electrodes is below a certain value.This value is frequently called the hold-off value. Spark gaps may beconstructed having hold-off values ranging from 500 to at least 30,000volts. The

trigger electrode is not energized while the hold-ofi value is beingdetermined or while the spark gap is being subjected to a specifiedhold-oft test.

Secondly, breakdown between the dome electrodes must occur when thepotential across them is greater than a certain minimum. potentialbetween the two dome electrodes is great enough, the gas therebetweenshould break down, and conduction bet-ween the electrodes should follow.The value at which the breakdown should occur (keeping in mind that thetrigger electrode is not energized at this time either) is often termedthe minimum static breakdown voltage. The static breakdown voltage ofspark gaps is usually not less than 25% greater than the hold-off value.

Thirdly, the breakdown must occur when the potential applied across thetwo dome electrodes is within a certain specified range between thehold-01f and static breakdown values when the trigger electrode isenergized. Furthermore, this breakdown must occur within a certain, andvery short, period of time after the trigger electrode is energized.This delay period between application of the pulse to the triggerelectrode and the breakdown between the two dome electrodes is calledthe delay time. This is usually a fraction of a microsecond. The averagetime variation that a spark gap tube may exhibit in its delay timeduring a series of pulses is called jitter.

Not only is the triggered spark gap a high voltage switch, but thecurrent that is switched can be considerable. Since it is an arcdischarge device, its conduction current is limited only by theconstants of the external circuit. Such operation results in very hightemperatures at the cathode dome electrode because of the high currentdensity, and also in intense ionic bombardment of the cathode because ofthe high voltages. These two effects result in an erosion of the cathodedome electrode due to the vaporization of the cathode metal as a resultof the high temperatures and also due to the ionic bombardment. There isa subsequent and inevitable deposition of the vaporized electrode metalelsewhere (and undesirably) on the interior surfaces of the spark gaptube. The combined result of high temperature and ionic bombardment onthe cathode dome electrode is, for our purposes here, termed sputtering.

A metallic coating on the insulating materials that separate the threeelectrodes within the spark gap aiiects the electrical characteristicsof the spark gap in an undesirable way. Typically, a thin conductivecoating forms on the insulator that separates the two dome electrodes,and also on the insulator that separates the trigger electrode with itscoupled dome electrode. The dome electrode triggered with the triggerelectrode acts as the cathode in normal operation and is usually calledthe cathode electrode or trigger dome electrode. The other of the twodome electrodes is termed the main dome electrode.

Clearly, the more often the tube is fired or pulsed, the more sputteringresults, and as time goes on, the greater the erosion of the triggerdome electrode and deposition of the eroded metallic material on theinsulation of the spark gap. In this way, sputtering not only causesunacceptable performance of the tube with respect to the hold-oi? valueand the delay time, but the life of the tube is greatly reduced. Theinventions described in the copending applications of Edward E.Hafkemeyer and Robert E. Hueschen for Arc Discharge Device, Serial No.87,048, filed December 29, 1960, and Robert E. Hueschen for TriggeredSpark Gap, Serial No. 79,445, filed Decernber 29, 1960, have efiectivelydecreased sputtering and its effects to a point where very satisfactoryoperation is obtained when no problems are encountered in In otherwords, if the difference of aoaaoas 3: terms of interelectroderesistance resulting from the process of constructing the tube itself.

Metallic and other conductive contaminants which may be deposited on theinside surfaces during the construction of the tube also contribute toundesirable performance and decreased life of the spark gap. For thisreason, each of the metallic and ceramic tube parts is subjected toseveral rigid cleaning processes before they are brazed together intheir final assembly. For example, each part is vacuum fired, withparticular attention to the ceramic elements to insure that all of theadsorbed and absorbed gases are driven off. In addition, each of theparts is ultrasonically cleaned with an appropriate detergent. Thesecareful cleaning procedures prior to assembly help to insure cleaningthe inner surfaces of the spark gap.

Even with these cleaning techniques in the fabrication of the tubes, andwith control of the problem of sputtering during the operation of thetube, more is required. Another problem was discovered. In theconstruction of the tube the brazing technique was responsible forrendering unsatisfactory many of the tubes produced. Construction of thetube involves brazing four separate and distinct joints. Despite thefact that the brazing is performed in a vacuum oven by radiated heat, itwas ascertained that deposits of conductive material had been generatedduring the brazing process which lined the inner surfaces of the tube. Aperplexing aspect is that the deposition of conductive material insidethe tube was due to vaporization of the brazing alloy because at leasttwo of the braze joints are heated to the melting point of the brazingmetal at slightly different times in the firing proc ess. This occurreddespite the fact that the distance between the two most widely separatedbraze joints is less than three-quarters of an inch. A difference offive seconds between the times at which the two joints melt can result,apparently, in sufficient metallic vaporization of the brazing metal torender the tube useless for practical purposes.

It is not fully understood why this time differential between themelting points at the different brazing joints occurs (especially inview of the fact that the brazing is done inside a vacuum oven, and theentire spark gap tube is so small). It has been ascertained, however,that a temperature gradient exists within the vacuum oven in the regionof the spark gap tube. This may explain the time differential betweenthe melting points. In addition it may, itself, independently contributeto the metallic deposition inside the tube as may be seen from thehereinafter to be presented explanations.

It is the primary object of this invention, therefore, to provide animproved method for constructing an arc discharge tube characterized bylonger life and better performance than has heretofore been possible,and which for any specified performance characteristic, may bemanufactured to satisfy those characteristics with a higher productionyield than has been heretofore possible.

It is another object of this invention to provide a method forconstructing an arc discharge tube which precludes the deposition ofelectrically conductive matter on the inside surface of the tube.

It is still another object of this invention to provide a method ofbrazing together parts of a spark gap tube with a minimum amount ofvaporization of the brazing metal.

In accordance with the principles of the invention, a method for brazingspark gap joints is followed such that there is substantially nometallic film deposited on the inside surfaces of the tube during thebrazing process. The brazing method in accordance with the inventionutilizes a two-step technique. The brazing joints that would ordinarilyfirst be heated to the melting point in the old one-step process, arebrazed during the first firing step of the two-step method. Advantage istaken express- 1y of the temperature gradient that has been found toexist in the oven across the tube. The trigger dome electrode which hasan aperture through its surface is located toward the lower end of thetemperature gradient. As a consequence, metallic vapor generated withinthe spark gap tube may pass through the aperture and out of the tube tobe deposited elsewhere on a cooler surface. After the first-step jointshave been brazed, the partially assembled spark gap tube is cooled inthe oven in the vacuum system. Then the remaining component tube partsare added, brazing metal is placed at the other two joints, and theentire spark gap tube placed in the oven for the second firing step. Thebrazing metal applied at the second-step joints has a lower meltingpoint than the brazing metal applied to the first-step joints.Therefore, the second-step joints are brazed without any danger of thebrazing metal of the first-step joints vaporizing during the second-stepfiring.

The novel method believed to be characteristic of the invention is setforth with particularity in the appended claims. The invention itself,however, as to its organiza tion, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings.

In the drawings:

FIGURE 1 is a perspective view with portions cut away of a triggeredspark gap tube constructed in accordance with the invention;

FIGURE 2 is a longitudinal cross section of the trig gered spark gap ofFIGURE 1 taken through a plane including the longitudinal axis of thetube;

FIGURE 3 is a longitudinal cross sectional view of the structure of thebrazing oven used in constructing the triggered spark gap tube inaccordance with the inven tion; and

FIGURE 4 is a plan view taken substantially along the line 44 of FIGURE3.

The Structure of the Spark Gap Tube Referring to FIGURES 1 and 2, thereis shown a triggered spark gap tube constructed in accordance with theprinciples of the invention. So that a frame of reference may be had,the external diameter of the cylindrical body insulator 11 of the tubeof FIGURE 1 would be characteristically approximately .675 of an inchfor a typical tube. The cylindrical insulating body of tube 11 ispreferably a ceramic material to which the metal electrodes may bebrazed. A particularly satisfactory type of ceramic comprisesninety-five percent A1 0 with the remaining five percent compisirng Cr OSiO MgO and (3210. A commecially avialable ceramic having thiscomposition goes under the trade name of Diamonite P3142-1, although weare not restricting the ceramic either to this type or to this chemicalcomposition. It need only satisfy the requirement that it be a goodinsulator, strong, and easily adaptable for brazing to metals such astantalum, molybdenum or Kovar.

The major functions of the ceramic body insulator 11 are: the formationof the envelope of the tube so that the gas may be contained therein ata desired pressure; and the electrical separation and insulation of thetwo dome electrodes 12 and 13. At the lower portion of the ceramic bodyinsulator 11 is a hollow hemispherical metallic electrode 12. This isthe main dome electrode and in its usual operation normally has appliedto it a positive potential relative to the dome electrode 13. Located atthe top portion of insulator 11 is the trigger dome electrode 13 whichis in many respects similar to main dome electrode 12, except that inthe central portion of the hemisphere, an aperture 14 is defined thereinfor purposes that will be described below.

Each of dome electrodes 12 and 13 has a collar forming a base to affordmeans for brazing to the bottom and top edges of the ceramic bodyinsulator 11. Disposed between the collar of main dome 12 and insulator11 is a thin washer 31 of brazing metal (copper) used for brazing thecollar of the electrode 12 to the bottom edge of the cylindrical bodyinsulator 11. In similar fashion, the collar of the trigger dome isbrazed to the top edge of the cylindrical insulator 11 through themedium of a thin metallic washer 32 of the same metal (copper) as thatused for securing electrode 12 to the ceramic body insulator 11.

Except for the aperture 14 located in the center of trigger dome 13, anda small hole 29 to permit easy loading' of gas during the gas fillingprocess, the unitary structure formed by the body insulator 11 and theelectrodes 12 and 13 would be air tight. The general configurationresulting is that the two hollow hemispheres 12 and 13 are disposedinside the hollow insulator 11, with the hollow hemispherical main dome12 being disposed concave downward and trigger dome 13 concave upward.The distance in the tube between the closest points or the surfaces ofdomes 12 and 13 determines the breakdown voltage of the tube (inconjunction with the type of gas filling the tube and its pressure).

Attached to the brazed collars of dome electrodes 12 and 13 respectivelyare electrical terminals 19 and 20 respecively, to which may be appliedthe potential which is to be switched by the spark gap.

Trigger dome electrode 13 and main dome electrode 12 are made of amaterial characterized by a high melting point and low vapor pressure.This is necessary, because when the spark gap is activated, its internaltemperature may reach to approximately 6,000 C. Tantalum is a metalwhich well satisfies these requirements and the additional requirementthat it be easily brazed to the ceramic insulator 11. Molybdenum,tungsten and colum-bium are examples of other metals which may be usedbecause they have similar properties.

The third electrode of the spark gap is the trigger electrode 21 (asdistinguished from the trigger dome electrode 13). It is disposed in theaperture 14 formed in trigger dome electrode 13. The trigger electrode21 is disposed coaxially with the cylindrical portion of insulator 11.Extending upwardly from the trigger electrode 21 is a trigger tube 22Within which the trigger electrode 21 fits. A ceramic cap insulator 23,with a hole through its center for receiving the trigger tube 22, ismounted on top of the spark gap, so that the rim portion of capinsulator 23 may be brazed to the top surface of the collar of triggerdome electrode 13. A thin washer-like element 33 of silver-copper isdisposed between the rim of cap 23 and the collar of dome 13 to providethe material necessary for brazing the two together. The hole throughthe center of ceramic cap insulator 23 is closed by brazing its boundaryto the outer periphery of trigger tube 22 with a thin wire 34 of thesame silver-copper brazing material. The brazing elements 33' and 34 ofthe silvercopper depressed solid solution has a melting point which islower thanthat of either silver or copper and therefore lower than thatof copper brazing elements 31 and 32. By virtue of the ceramic cap 23being brazed at these areas, the entire volume within the spark gap bodyitself is air tight. This volume is filled with nitrogen at a pressureof from approximately one-half of an atmosphere to one atmopshere. Theprecise pressure of the nitgrogen (in conjunction with the spacingbetween the trigger dome 13 and the main dome electrode 12) determinesthe breakdown potential of the spark gap. Other gases and combinationsof gases, such as helium/nitrogen and krypton 85/nitrogen may be used.

Trigger electrode 21 is preferably of tungsten, which also has a lowvapor pressure and high melting point. Ceramic cap insulator 23 may beof the same material as the body insulator 11. The trigger tube 22 ispreferably of Kovar or nickel, although materials such as copper orcombinations such as copper/Kev ar may be used.

Although trigger electrode 21 is disposed in the aperture 14 of triggerdome electrode 13, it does not fill the 6v aperture. Surrounding triggerelectrode 21 and contiguous thereto, and otherwise filling the aperture14, is an insulator '24. Trigger insulator 24 is a ceramic materialwhich may be of the same type as the body insulator 11 and/ or ceramiccap insulator 23.

The Structure of the Brazing Oven With the presentation of the structureof the spark gap completed, a description of the apparatus used forbrazing the component parts of the spark gap tube together isappropriate. In FIGURE 3, there is disclosed a vacuum oven used for thebrazing process. Mounted upon a platform 41 which has an exhaustaperture 42 therethrough, is a large bell jar 43 with the bottom rim ofthe bell jar resting on the platform such that exhaust aperture 42 isenclosed. The rim of bell jar 44 and platform 41 constitute an airtightjoint by virtue of appropriate vacuum gasket material disposedtherebetween. Underneath platform 41, and in communication with exhaustaperture 42, is a vacuum system comprising a standard vacuum pump withappropriate connections to exhaust aperture 42. The standard vacuumsystem is represented generically by the block 44. It should be ofappropriate size and capacity for maintaining the inside volume of j thebell jar under a vacuum of approximately 5 l() millimeters of mercury.

Coaxial with and surrounding bell jar 43, is a radiofrequency coil '45whose function is to provide inductive heating for the oven. Coil 45 isconnected to a source of radio-frequency energy 46. Typically for anoperation of this type, the frequency may be 480 kilocycles per secondfrom a source capable of producing 15 kilowatts of power. A thin walled,hollow cylindrical molybdenum or tantalum susceptor 47 mounted withinjar 43 operates to radiate heat within the vacuum of the bell jar.Susceptor 47 is in turn thermally excited by energy induced therein fromthe RF coil 45. The combination of tantalum susceptor 47 with itsexternal RF energizing coil 45 is capable of producing temperatures ofat least 1100 C. within the bell jar within a relatively short period oftime. Susceptor 47 is supported within jar 43 upon ceramic fixturingmaterial. Thus, susceptor 47 is mounted upon a ceramic block 48,preferably of a porous alumina, and which in turn is supported by acolumn of the same material resting upon the platform 41.

Disposed within the hollow defined by cylindrical susceptor 47 andresting upon a small fixture 49 (also of alumina) are eight spark gaptubes 50, which are to be brazed in the oven. Spark gap tubes 50 areeach identically the same as the tube of FIGURES 1 and 2 describedabove, although during the first firing step in the oven none of thespark gap tubes is completely assembled. The apparatus for holding eachof the spark gap tubes in place in the oven will be described atappropriate points below in the explanation of the brazing method.

The Brazing Method Having described the spark gap tube and the oven forbrazing the component parts of the tube, we proceed to a description ofthe brazing method in accordance with the principles of the invention.vReference will often be made in the following description to thecomponent parts of the spark gap tubes 50, which are depicted incross-section in FIGURE 2.

The brazing process is performed in two separate parts. The first partinvolves a first heating or firing step of certain component elements ofthe tube so as to make two brazed joints. After this first firing step,the tube is cooled in an argon atmosphere, which is added to the oven,to hasten cooling. Then, as the second part of the process, certainother components of the tube in combination with a different brazingmaterial are added on to the previously brazed structure and the entirestructure is reinserted into the oven. The heating process is repeatedto complete the last two joints.

act-aces The first firing step will now be described. The ceramiccylindrical insulator body 11 of the spark gap tube, in conjunction withthe dome electrodes 12 and 13, are to be brazed together through themedium of the copper brazing metal washers 31 and 3'2. Thus, thesecomponents are mounted in the same relationship to each other as shownin FlGURE 2, with a titanium hydride slurry applied to the jointswherein copper washers 31 and 32 are disposed. In this first firing steptrigger electrode 21, trigger tube 22, trigger insulator 24, and ceramiccap 23 take no part and are in no way involved. However, a hollowceramic cylinder 51 much like, but longer than, ceramic insulator body11 is placed over the collar of the trigger dome 13. A metal, solidcylindrical weight 52 (which may be of stainless steel or copper) ismounted on top of cylinder 51. Weight 52 has a shoulder cut around theedge of its bottom face so that it is seated securely in the top openingof cylinder 51. A flat metal disc 53 (also stainless steel or copper) isthen placed over the top surfaces of weights 52 so as to align all eightspark gaps. The weights 52 serve to press the components of each sparkgap together for the brazing process. The disc 53 further assures thatthe spark gap tube will be maintained in position during the process. Itis desirable in this connection to place a small bore into the top ofeach of weights 52 and to add eight mating protruding pins from thebottom of disc 53.

The top portion of cylinder 51, the weights 52, and disc 53, as they aremounted, extend vertically higher than the top layer of RF coil 45.

The tube structures described for the first step are then mounted inhell jar =43 on the alumina disc 4? within the hollow of susceptor 47.Coil 45 is then energized such that the inside of bell jar 43, Le, thesusceptor, is heated gradually to a temperature of approximately 1100"C. and maintained at that temperature for a brief period (approximatelyfifteen seconds).

After the susceptor has reached its appropriate first-step temperatureof approximately 1100" C., brazing occurs. With the spark gap tube partsso heated, the collars of the tantalum domes 12 and 13, as well as theedges of the ceramic cylindrical insulator 11 are at a high enoughtemperature such that they are wetted by the melted brazing alloy formedby the copper and the titanium from the titanium hydride. The totalheating period is twenty-five minutes. RF power is then turned off andcooling occurs for ten minutes, under vacuum. Then argon is added to thesystem to hasten cooling. After one hour, the parts are removed.

The weighting and mounting elements 51-53 are then removed and theremaining components of the triggered spark gap tube are placed inposition for the second firing step. Thus, \igger electrode 21 andtrigger insulator 24 are inserted into their appropriate aperture 14- inthe trigger dome electrode 13. Trigger cap 23 is also placed in positionwith trigger tube 22 appropriately located within its central aperture.

At the edges of the cap 23, and contiguous thereto as well as to the topsurface of the collar of trigger dome electrode 13, is placed asilver-copper braze washer 33. This region constitutes the third of thefour joints that require brazing. The fourth joint is the region whereinthe periphery of the trigger tube 22 meets the boundary of the apertureof ceramic cap 23. At that region near the top surface of cap 23 isdisposed a silver-copper braze wire 34. Titanium hydride slurry isapplied to the third and fourth joints.

The completely assembled spark gap tubes are then placed in the oven andmounted as previously. The susceptor is heated during the second firingstep to a temperature of approximately 975 C., which is about 125 C.less than its temperature during the first firing step. Thesilver-copper eutectic alloy which is used, has a meltting point belowthat of either silver or copper. With the susceptor at this temperature,the spark gap tubes within are subjected to a lower temperature, and inparticular lower than that at which copper washers 31-32 (now alloyedwith titanium) of the first joint can melt; there can be no melting ofor vaporization from the first two brazed joints (with the pressurewithin the oven the same as it was during the first firing step). Thistemperature, however, is sufficient to melt the silver-copper alloy.After the third and fourth joints are brazed in this manner with thesilver-copper brazing material (the 975 C. temperature of the susceptoris also held for a short time, erg, fifteen seconds, with the totalheating time being twenty-five minutes as before), the RF power isremoved and the spark gap tubes permitted to cool. They are then removedand are ready to be exhausted and filled with nitrogen at theappropriate pressure.

An important feature of this two-step process may now be comprehended,and its considerable advantages over a one-step process more fullyappreciated. It may be recalled that during the first firing step, aceramic cylinder 51 and metal weight 52 were placed over the partialspark gap structure during mounting to apply pressure during the brazingprocess. It has been found that the inner surface of cylinder 51 and thebottom face of weight 52 become coated with deposits of copper as aresult of the first firing step. These deposits of copper come fromcopper vapor generated within the volume of the spark gap tube. Thecopper vapor which is generated rises through aperture 14- of triggerdome electrode 13. Now, it may be understood, that if prior art one-steprather than two-step firing is utilized, the trigger dome electrodeaperture would be filled with trigger electrode l4- and triggerinsulator 2 s, with the result that the trigger electrode and itsinsulator would be coated with the copper deposit. When using thetwo-step method, therefore, it may be seen to be desirable to mount thespark gap tube in the oven (at least during the first firing step), suchthat trigger dome electrode 13 with its aperture 14- therein is at thetop. In this way, the aperture 14 may act as a type of chimneypermitting vaporized copper to pass therethrough'with relatively littledeposition on any of the other surfaces.

The question may well be asked as to why the vaporized copper does infact rise in the spark gap tube in this manner. It is believed that atemperature differential exists between the bottom of the spark gap tubeand the elements 5153 which may account for the fact that the coppervapor rises through aperture 14 of dome electrode 13. Since theweighting and mounting elements 5153 extend verticaly higher thansusceptor 47, they are in a cooler region of the oven than t' e sparkgaps within the internal volume of the susceptor. As a consequence, anycopper vapor that may be generated within the spark gap during the firstfiring step tends to pass along a straight line up through aperture 14toward elements 51-53. These latter elements being cooler than thosewithin the hotter region below provide a surface upon which the hotcopper vapor can condense.

A possible reason for excessive vaporization during the prior artone-step firing process is related to the geometries of the brazingjoints and metal elements. Thus, if solely one type of metal is appliedat all of the joints in a one-step brazing process, the differentgeometries (i.e., washer 31 and wire ring 34 of the brazing elements)may possibly be responsible for a time differential in melting points.

An additional possible explanation for the melting point timedifferential in the prior art, one-step brazing operation has to do witha possible horizontal temperature gradient between the region of thespark gap tube nearest to susceptor 47 and those regions of the sparkgap tube more remote therefrom. Thus, for example, for a spark gap tubemounted in the oven on the righthand side thereof, the right-hand edgesof the brazing washers 3-1-63 (in a one-step operation) would be heatedmore rapidly than the left-hand side of those washers. In addition, theright-hand edges of those three washers are closer to the susceptor andtend to be heated more rapidly to higher te.iiperatures than the brazingwire or ring 34 disposed around trigger tube 22 and centrally located onthe spark gap tube. The brazing metal 34, therefore, is more remote fromthe tantalum susceptor than the edges of the brazing washers 33l-33,irrespective of whether the tube is mounted on the right-hand or theleft-hand side of the oven, or in the center thereof. Presumably,therefore, the washers 31-33 commence melting, at least at some edges,in advance of any portion of the wire 34.

All of the above factors may contribute individually or in combinationto account for the excessive metallic deposits within the spark gapcharacteristic of a one-step brazing operation.

Although the brazing elements 31 and 32 were described as being copper,other brazing metals have also been found to provide excellent results.The main requirement for the metal of washers 31 and 32 is that it meltat a higher temperature than that of the brazing elements 3 3 and 34. Ithas been found that a goldcopper eutectic in the ratio of fifty percentgold to fifty percent copper provides an excellent brazing metal whichwill melt at a temperature higher than that of the silvercopper used forelements 33-34. The silver-copper is preferably in proportions ofseventy-two percent silver to twenty-eight percent copper. This eutectichas a melting point of 779 C. at 5X millimeters of mercury.

While we have shown particular embodiments of our invention, it will beunderstood that many modifications may be made without departing fromthe spirit thereof, and we contemplate by the appended claims to coverany such modifications as fall within the true spirit and scope of ourinvention.

What we claim is:

1. The method of constructing a spark gap tube having a given number ofmetallic and ceramic parts forming a plurality of joints to be brazedcomprising the steps of: assembling a plurality, but less then all, ofsaid given number of parts to form at least one of said joints; placingsaid given number of parts in an induction heating oven and forming avacuum in said oven; brazing said one joint with a first brazing metalhaving a given melting point; assembling the remainder of said givennumber of parts upon the previously brazed parts to form at least asecond joint spaced from said first joint by a distance of approximatelyan inch or less; and brazing said second joint with a second brazingmetal having a lower melting point than that of said first brazingmetal.

2. The method of constructing a spark gap tube having a given number ofmetallic and ceramic parts forming a plurality of joints to be brazedcomprising the steps of: assembling a plurality, but less than all, ofsaid given number of parts to form at least one of said joints; placingsaid given number of parts in an induction heating oven and forming avacuum in said oven; brazing said one joint with a first brazing metalof copper; assembling the remainder of said given number of parts uponthe previously brazed parts to form at least a second joint spaced fromsaid first joint by a distance of approximately an inch or less; andbrazing said second joint with a second brazing metal of silver andcopper in solid solution.

3. The method of constructing a spark gap tube having two metallic domeelectrodes and a ceramic body insulator for spatially and electricallyisolating said dome electrodes from each other comprising the steps of:form mg an aperture through one of said electrodes; assembling said domeelectrodes and said body insulator together to form a volume that wouldbe closed except for said aperture; placing brazing metal between saidelectrodes and said ceramic insulator so that each of said electrodescan be brazed to said insulator; placing the assembly of saidelectrodes, insulator and brazing metal within a brazing oven withinwhich a temperature gradient exists when the temperature of said oven israised; disposing said assembly within said oven with said aperturedelectrode closer to the lower temperature end of said gradient than theother of said electrodes; forming a vacuum within said oven and brazingsaid electrodes and said insulator by raising the temperature withinsaid oven to the melting point of said brazing metal at said vacuum.

4. The method of constructing a spark gap tube having two metallic domeelectrodes and a ceramic body insulator for spatially and electricallyisolating said dome electrodes from each other comprising the steps of:forming an aperture through one of said electrodes; assembling said domeelectrodes and said body insulator together to form a volume that wouldbe completely closed except for said aperture; placing brazing metalbetween said electrodes and said ceramic insulator so that each of saidelectrodes can be brazed to said insulator; placing the assembly of saidelectrodes, insulator and brazing metal within a brazing oven withinwhich a temperature gradient exists when the temperature of said oven israised; disposing said assembly within said oven with the axis of saidaperture through said electrode parallel to the direction of saidtemperature gradient; forming a vacuum within said oven and brazing saidelectrodes to said insulator by raising the temperature Within said ovento the melting point of said brazing metal at said vacuum.

5. The method recited in claim 4 including the steps of brazing anadditional element to said brazed assembly with another brazing metaldifferent from said brazing metal.

6. The method as recited in claim 5 wherein said other brazing metal hasa melting point lower than that of said brazing metal at the samepressure.

7. The method of constructing a spark gap tube having two metallic domeelectrodes and a ceramic body insulator for spatially and electricallyisolating said dome electrodes from each other comprising the steps of:forming an aperture through one of said electrodes; assembling said domeelectrodes and said body insulator together to form a volume that wouldbe completely closed except for said aperture; placing brazing metalbetween said electrodes and said ceramic insulator so that each of saidelectrodes can be brazed to said insulator; placing the assembly of saidelectrodes, insulator and brazing metal within a brazing oven withinwhich a temperature gradient exists when the temperature of said oven israised; disposing said assembly within said oven with the axis of saidaperture through said electrode vertically oriented and with saidapertured electrode above said other dome electrode; forming a vacuumwithin said oven and brazing said electrodes to said insulator byraising the temperature within said oven to the melting point of saidbrazing metal at said vacuum.

References Cited in the file of this patent UNITED STATES PATENTS1,848,408 Bauer Mar. 8, 1932 2,411,301 Stanitz Nov. 19, 1946 2,503,429Ziegler Apr. 11, 1950 2,677,877 Cox May 11, 1954

